ABOARD THE U.S.S. GEORGE WASHINGTON — Fighter jets roared off an American aircraft carrier into a gray sky on Wednesday as the United States kicked off a joint naval exercise off the west coast of the Korean Peninsula, with the South Korean president, Park Geun-hye, calling for strong punishment if North Korea initiated a military provocation.

The North Korean leader, Kim Jong-un, has raised tensions on the divided peninsula in recent weeks by launching a string of rockets and missiles, including its Scud-type ballistic missiles, even as his government took steps toward a possible thaw in relations, like a recent agreement to send a delegation and cheering squad to the Asian Games in South Korea this fall.

Mr. Kim has personally supervised some of these missile tests, which took place close to the border with the South, according to South Korean officials and the North Korean news media. While some leaders were guessing at his intentions, some South Korean analysts feared that the North might attempt military provocations to raise tensions unless the South relaxed its hard-line policy and engaged the North with dialogue and possible economic concessions.

Speaking to a group of South Korean military leaders, Ms. Park called the North Korean government so unpredictable that it was impossible to foresee its behavior “even an inch ahead.” “If they launch a provocation, I ask you, commanding officers of the military, to retaliate with a strong initial countermeasure,” Ms. Park said Wednesday.

She also cited international concern that North Korea might attempt another nuclear test.

About 200 nautical miles off the west coast of South Korea, the United States aircraft carrier George Washington cruised through drizzle as F/A-18 Super Hornet fighter jets screamed off the deck.

Officers on board said that the jets conducted 108 sorties from the U.S.S. George Washington on Wednesday alone in an exercise that Rear Adm. Mark C. Montgomery said was designed to bolster the American and South Korean forces’ combined “ability to counter maritime special operations force insertions.”

In recent years, Mr. Kim has overseen the landing drills of North Korea’s armed forces, involving hovercraft, as well as test-firings of its multiple-rocket launchers and artillery amassed near the border with the South.

The United States Navy’s five-day joint drill with South Korea involves six American ships, a large number of South Korean Navy vessels and United States and South Korean Air Force combat aircraft, both in the seas off the east and west coasts of South Korea, said Admiral Montgomery, commander of the United States Seventh Fleet’s Carrier Strike Group Five operating out of Yokosuka, Japan.

Last Saturday, North Korea called the joint drill “another challenge” against its recent overture to try to ease relations with the South.

It called the American warships “a fleet of pirates” with “nuclear strike capabilities” and vowed to strengthen its own “self-defense nuclear power.” The country accused Washington of spoiling a possible thaw between the two Koreas and of coaxing both Japan and South Korea into its “hegemonic” design to contain China and North Korea.

The United States-South Korea exercise will be followed by a trilateral naval drill involving the Japanese maritime self-defense forces.

That two-day search-and-rescue exercise, which is scheduled to begin Monday, will be the first of its kind since Japan reinterpreted its pacifist Constitution to allow its military a more assertive role to help defend allies, particularly the United States.

Tokyo’s decision has led to misgivings in the region where Japan is locked in tense territorial standoffs with a number of countries. In South Korea, where anti-Japanese sentiments run deep as a legacy of Japan’s colonial rule in the early 20th century, many fear that Prime Minister Shinzo Abe will lead Japan back to militarism.

But Washington welcomed Japan’s so-called collective self-defense, calling for closer military cooperation among the United States, Japan and South Korea.

A version of this article appears in print on , on Page A7 of the New York edition with the headline: South Korea: Joint Exercises Begin. Order Reprints | Today’s Paper | Subscribe